Integrated circuit (IC) printing is an emerging technology that attempts to reduce the costs associated with IC production by replacing expensive lithographic processes with simple printing operations. By printing an IC pattern directly on a substrate rather than using the delicate and time-consuming lithography processes used in conventional IC manufacturing, an IC printing system can significantly reduce IC production costs. The printed IC pattern can either comprise actual IC features (i.e., elements that will be incorporated into the final IC, such as the gates and source and drain regions of thin film transistors, signal lines, opto-electronic device components, etc.) or it can be a mask for subsequent semiconductor processing (e.g., etch, implant, etc.).
Typically, IC printing involves depositing a print solution (generally an organic material) by raster bitmap along a single axis (the “print travel axis”) across a solid substrate. Print heads, and in particular, the arrangements of the ejectors incorporated in those print heads, are optimized for printing along this print travel axis. Printing of an IC pattern takes place in a raster fashion, with the print head making “printing passes” across the substrate as the ejector(s) in the print head dispense individual droplets of print solution onto the substrate. At the end of each printing pass, the print head makes a perpendicular shift relative to the print travel axis before beginning a new printing pass. The print head continues making printing passes across the substrate in this manner until the IC pattern has been fully printed.
Once dispensed from the ejector(s) of the print head, print solution droplets attach themselves to the substrate through a wetting action and proceed to solidify in place. The size and profile of the deposited material is guided by competing processes of solidification and wetting. In the case of printing phase-change materials for etch mask production, solidification occurs when the printed drop loses its thermal energy to the substrate and reverts to a solid form. In another case, colloidal suspensions such as organic polymers and suspensions of electronic material in a solvent or carrier are printed and wet to the substrate leaving a printed feature. The thermal conditions and material properties of the print solution and substrate, along with the ambient atmospheric conditions, determine the specific rate at which the deposited print solution transforms from a liquid to a solid.
If a first droplet and a second adjacent droplet are applied onto the substrate within a time prior to the phase transformation of the first droplet, the second droplet will wet and coalesce to the first droplet in its liquid or semi-liquid state to form a continuous printed feature. Print solutions having high surface tension will also beneficially prevent the next overlapping droplet from spreading on the substrate surface, thus minimizing the lateral spreading of the droplets. FIG. 1a shows a photograph of a printed feature 100a that was printed in a single printing pass in the X axis direction. Because adjacent droplets deposited during the single printing pass did not have time to dry between ejection events, feature 100a exhibits the desired homogeneity and smooth side wall profiles that result from optimal droplet coalescence. In contrast, FIG. 1b shows a photograph of a printed feature 100b formed by raster printing in the Y axis direction. Feature 100b therefore represents a “multi-pass” feature; i.e., a printed feature formed by multiple passes of the print head. In a multi-pass feature, the droplets deposited during sequential passes of the print head are typically dry before any adjacent droplets from the next printing pass are deposited. Consequently, the drops of print solution that make up the multi-pass feature are not able to coalesce and therefore create “scalloped” feature borders. This edge scalloping can be seen in FIG. 1b, as the individual print solution droplets 110b used to form feature 100b are all clearly visible.
Typically, an IC pattern includes both multi-pass features and features that are aligned with the print direction. FIG. 1c shows a photograph of an IC pattern 100c printed using a conventional IC printing process—in this case a raster printing operation in the Y axis direction. IC pattern 100c is made up of an array of transistor elements 120 interconnected by multiple address lines 160 and word lines 170. Word lines 170, which run parallel to the Y axis and were therefore aligned with the print direction, exhibit the desirable homogeneity and smooth sidewalls described with respect to FIG. 1a. However, address lines 160, which are printed by multiple printing passes in the Y axis direction, all exhibit the undesirable edge scalloping and non-coalescence described with respect to FIG. 1b. 
The edge scalloping shown in FIGS. 1b and 1c is related to a variety of problematic issues. For example, if the IC pattern is a mask, the irregular edges of feature 100b can result in unreliable print quality and patterning defects leading to inconsistent device performance. Perhaps more significantly, edge scalloping in an actual IC feature indicates a potentially serious underlying defect. The electronic behavior of an IC feature is affected by its molecular structure. In particular, the molecules of organic printing fluids are typically long chains that need to self-assemble in a particular order. However, if a droplet of such printing solution solidifies before an adjacent droplet is deposited, those chains are not allowed to properly assemble, leading to a significant reduction in the electrical continuity between the two droplets. This in turn can severely diminish the performance of the device that incorporates the scalloped printed feature.
What is needed is a system and method for accurately printing IC patterns that allows the printed features to be optimized for edge profile and electrical continuity.